Implantable stimulation devices deliver electrical stimuli to nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder subluxation, etc. The description that follows will generally focus on the use of the invention within a Spinal Cord Stimulation (SCS) system, such as that disclosed in U.S. Pat. No. 6,516,227. However, the present invention may find applicability with any implantable medical device or in any implantable medical device system.
An SCS system typically includes an Implantable Pulse Generator (IPG) 10 shown in plan and cross-sectional views in FIGS. 1A and 1B. The IPG 10 includes a biocompatible device case 30 that holds the circuitry and battery 36 necessary for the IPG to function. The IPG 10 is coupled to electrodes 16 via one or more electrode leads 14 that form an electrode array 12. The electrodes 16 are configured to contact a patient's tissue and are carried on a flexible body 18, which also houses the individual lead wires 20 coupled to each electrode 16. The lead wires 20 are also coupled to proximal contacts 22, which are insertable into lead connectors 24 fixed in a header 28 on the IPG 10, which header can comprise an epoxy for example. Once inserted, the proximal contacts 22 connect to header contacts 26, which are in turn coupled by feedthrough pins 34 through a case feedthrough 32 to circuitry within the case 30.
In the illustrated IPG 10, there are thirty-two lead electrodes (E1-E32) split between four leads 14, with the header 28 containing a 2×2 array of lead connectors 24. However, the number of leads and electrodes in an IPG is application specific and therefore can vary. In a SCS application, the electrode leads 14 are typically implanted proximate to the dura in a patient's spinal cord, and when a four-lead IPG 10 is used, these leads are usually split with two on each of the right and left sides of the dura. The proximal electrodes 22 are tunneled through the patient's tissue to a distant location such as the buttocks where the IPG case 30 is implanted, at which point they are coupled to the lead connectors 24. A four-lead IPG 10 can also be used for Deep Brain Stimulation (DBS) in another example. In other IPG examples designed for implantation directly at a site requiring stimulation, the IPG can be lead-less, having electrodes 16 instead appearing on the body of the IPG for contacting the patient's tissue.
As shown in the cross section of FIG. 1B, the IPG 10 includes a printed circuit board (PCB) 40. Electrically coupled to the PCB 40 are the battery 36, which in this example is rechargeable; other circuitry 50a and 50b coupled to top and bottom surfaces of the PCB; a communication coil 42 for wirelessly communicating with an external controller (not shown); a charging coil 44 for wirelessly receiving a magnetic charging field from an external charger (not shown) for recharging the battery 36; and the feedthrough pins 34 (connection not shown). If battery 36 is permanent and not rechargeable, charging coil 44 would be unnecessary. (Further details concerning operation of the coils 42 and 44 and the external devices with which they communicate can be found in U.S. patent application Ser. No. 14/470,854, filed Aug. 27, 2014).
FIGS. 2A and 2B show some of the other circuitry 50a or 50b present in the IPG 10, and in particular FIG. 2A shows current distribution circuitry used to establish a current at any of the lead electrodes 16, which currents may comprise uni-phasic or multi-phasic current pulses. In this example, the current distribution circuitry includes a number of current sources (PDACs 52) and current sinks (NDACs 54). The PDACs 52 and NDACs 54 comprise Digital-to-Analog converters (DACs) able to respectively source and sink a current pulse of a desired amplitude Iout, which amplitude may be controllable in accordance with digital control signals (not shown). (“PDAC” and “NDAC” are so called because they are typically made from P-channel and N-channel transistors respectively). The sourced or sunk current from one or more active PDACs 52 or NDACs 54 is directed to selected electrodes 16 via switch matrices 56P and 56N, which are also digitally controlled (not shown). Note that the current distribution circuitry in this example also supports selection of the conductive case 30 as an electrode (Ecase 30), which case electrode 30 is typically selected for monopolar stimulation, as is well known. DACs 52 and 54 can also comprise voltage sources. The sourcing components (PDACs 52, matrix 56P) and the sinking components (NDACs 54, matrix 56N) can individually also be considered as current distribution circuitry.
Proper control of the DACs 52 and 54 and the switching matrices 56 allows any of the electrodes 16 to act as anodes or cathodes to create a current through a patient's tissue, R, hopefully with good therapeutic effect. In the example shown, PDAC 522 is controlled to source a current of amplitude Iout to anode electrode E1 via switch matrix 56P, while NDAC 541 is controlled to sink a current of amplitude Iout from cathode electrode E2 via switching matrix 56N. The DACs 52 and 54 can also be programmed to issue the current pulses with a particular frequency and duration, although this detail is unimportant to the present disclosure. Power for the current distribution circuitry is provided by a power supply voltage V+, as described in further detail in U.S. Patent Application Publication 2013/0289665 for example. More than one anode electrode and more than one cathode electrode may be selected at one time, and thus current can flow between two or more of the electrodes 16.
Other current distribution circuitries can also be used in IPG 10. For example, in an example not using switching matrices 56, each electrode Ei can be provided with a dedicated PDAC 52i and NDAC 54i, such as is disclosed in U.S. Pat. No. 6,181,969 for example. In another example, the PDACs 52 and NDACs 54 may provide currents of fixed amplitudes, with multiple of these DACs being selected by the switching matrices 56 to provide a sum of their currents at a selected electrode, such as described in U.S. Patent Application Publications 2007/0038250 and 2007/0100399.
Much of the current distribution circuitry of FIG. 2A, including the DACs 52 and 54 and the switch matrices 56, can be integrated on an Application Specific Integrated Circuit (ASIC) 60, as shown in FIG. 2B. In the example shown, ASIC 60 contains circuitry to support sixteen lead electrodes 16 and the case electrode 30. In a 32-electrode IPG 10, two such ASIC 60a and 60b are used, with the case electrode 30 being activated in only one of the ASICs 60a or 60b, in effect creating a 33-electrode device. ASICs 60a and 60b can be identically fabricated, and controlled by a microcontroller integrated circuit 62 (FIG. 3) acting as their master, as disclosed in U.S. Patent Application Publications 2012/0095529, 2012/0092031, and 2012/0095519. ASICs 60a and 60b may also contain other circuitry useful in the IPG 10, such as battery charging and protection circuitry (for interfacing off chip with the battery 36 and charging coil 44), telemetry circuitry (for interfacing off chip with telemetry coil 42), various measurement circuits, etc.
Also shown in FIGS. 2A and 2B are DC-blocking capacitors 70 (C(dc)) coupled to each of the electrode current paths 64 between the electrodes nodes Ei(a) present at the ASICs 60a/b (i.e., the outputs of the current distribution circuitry) and the electrodes 16 (Ei). The DC-blocking capacitors 70 are placed in series in the electrode current paths 64, and act as a safety measure to prevent DC current injection into the patient, and are commonly used in IPGs.
Given the amplitudes of the currents (Iout) typically provided to the electrodes 16 for effective therapy (on the order of milliAmps), and given the desire to prevent large voltages from building up across the DC-blocking capacitors 70 when passing such currents, the DC-blocking capacitors 70 have relatively large capacitance values—typically on the order of 1-10 microFarads. The DC-blocking capacitors 70 are thus relatively large in physical size, and can take up considerable space within the case 30 of the IPG 10, and in particular on the IPG's PCB 40.
This is shown in FIG. 3 for the 32-electrode IPG 10 described earlier, which shows the bottom surface of PCB 40 to which the charging coil 44 is mounted. Also mounted to the PCB 40 within the charging coil 44 are the ASICs 60a and 60b (each supporting 16 electrodes, and one supporting the case electrode 30), the master microcontroller 62, and the DC-blocking capacitors 70. As one skilled in the art will appreciate, the PCB 40 would contain other components as well, but these are not shown for simplicity. The DC-blocking capacitors 70 comprise typical surface-mountable ceramic capacitors, each of which may have a footprint or area of 120×60 mils (what is known in the art as a “1206” capacitor), 80×50 mils (an “0805” capacitor), or smaller.
Taken together, the DC-blocking capacitors 70 take up a relatively large area 75 on the bottom surface of the PCB 40, which may be as high as 30% of the total area of that surface (typically on the order of 10 cm2). This is unfortunate, because the DC-blocking capacitors 70 increase the size of the PCB 40, which increases the size of the case 30 and the IPG 10, which is preferably kept as small as possible to ease implantation surgery and promote patient comfort. The problem of the size of the DC-blocking capacitors 70 is further exacerbated by the industry's desire to provide IPGs with greater number of electrodes to provide patients with finer resolution and more-complex stimulation therapies. While at least some of the DC-blocking capacitors 70 could theoretically be placed on the other (top) surface of the PCB 40, this is not always an option: for example, in the example of IPG 10, the top surface of the PCB 40 is already fully occupied by the communication coil 42, the battery 36, and other surface-mounted circuitry 50a, as shown in FIG. 1B.
The prior art has recognized the problem of relatively-large DC-blocking capacitors 70 in a multi-electrode IPG. However, known solutions to this problem seek to minimize the number of DC-blocking capacitors 70 used. For example, in U.S. Pat. No. 7,881,803, an IPG is provided having only a single DC-blocking capacitor, which saves room in the IPG and hence allows for IPG miniaturization. U.S. Patent Application Publication 2010/0268309 likewise proposes an IPG with a reduced number of DC-blocking capacitors, i.e., less than the number of electrodes the IPG supports.
However, merely reducing the number of DC-blocking capacitors 70 as suggested by the art is not viewed by the inventor as a sufficient manner for addressing this problem. If too many DC-blocking capacitors 70 are removed compared to the number of electrodes the IPG supports, difficulties can arise. Therapy can be compromised, as the ability to freely choose electrodes operable as the anodes or cathodes may be restrained, because certain selectable current paths may not include a DC-blocking capacitor 70 as desired for safety; additional design complexities of the current distribution circuitry must be undertaken to address this concern. By contrast, if too few DC-blocking capacitors 70 are removed compared to the number of IPG electrodes 16, space savings in the IPG may be insignificant.
The problem of component size in an IPG is further exacerbated by newer advents that seek to add additional components to the IPG. For example, in U.S. Patent Application Publication 2014/0155970, which is incorporated herein by reference in its entirety, it is taught to use additional Electromagnetic Interference (EMI) filtering capacitors 71 (C(f)) in the electrode current paths 64, as shown in FIGS. 4A and 4B. These filtering capacitors 71 are coupled to the electrode current paths 64 in parallel between each of the electrode nodes Ei and a reference voltage (e.g., ground, such as the battery 36's negative terminal). As taught in the '970 Publication, filtering capacitors 71 are useful to shunt EMI coupled to the lead wires 20 to ground, such as the 64 MHz or 128 MHz frequencies typically present in a Magnetic Resonance Imaging (MRI) machine, and to hinder EMI from conducting to the conductive case 30. This reduces heating, and helps to protect the IPG circuitry from damage and from inadvertently stimulating the patient, thus making the IPG safer for use with patients requiring MRI procedures. Although not shown in FIGS. 4A and 4B, the '970 Publication teaches that the filtering capacitors 71 could also be coupled on the other side of the DC-blocking capacitors 70, i.e., in parallel between current distribution circuitry outputs Ei(a) and ground.
Also disclosed in the '970 Publication is the use of EMI filtering inductors 72 (Li), which like the DC-blocking capacitors 70 can placed in series in the lead electrode current paths 64, as shown in FIG. 4A, which filtering inductors Li can range from 0.5 to 3.0 microHenries. Alternatively, a single filtering inductor (Lcase) can be placed in the electrode current path of the case 30 having a value in the range of 50 to 200 nanoHenries, as shown in FIG. 4B.
The inventor recognizes that the '970 Publication discloses filtering capacitors 71 with values generally in the range of a few nanoFarads, which would be smaller in both capacitance and size when compared to the DC-blocking capacitors 70. For example, the inventor recognizes that filtering capacitors 71 could be implemented in an IPG 10 as 0805, 0603, or 0402 surface-mountable capacitors. Nonetheless, if filtering capacitors 71 are to be incorporated into IPG 10, space for such components must be found somewhere on the PCB 40 of the IPG 10, which space is already in short supply as noted earlier. If filtering inductors 72 are additionally incorporated into IPG 10, especially in each of the lead electrode current paths (Li; FIG. 4A), still more space in the IPG 10 would be needed.
A solution is therefore needed to accommodate these various electrode current path components, preferably in a manner that does not impact IPG flexibility in terms of the electrodes that can safely be chosen for patient therapy. Such solutions are provided in this disclosure.